1. Field of the Invention
The present invention relates generally to electronic signal conditioning elements and particularly to an improved design and method of manufacturing a signal filtering and conditioning assembly, which may be adapted for use in, inter alia, digital subscriber line (DSL) applications.
2. Description of Related Technology
With the increasing use of DSL (Digital Subscriber Line) and home networking technologies, it is often desirable to have DSL signals, home networking signals, or both present on a home telephone wiring network simultaneously with voice-band signals. Voice-band signals are commonly referred to as POTS (Plain Old Telephone Service) signals. Providing DSL service, home networking, and POTS over standard telephone lines permits the home telephone wiring network to operate effectively as a local area network (LAN), while at the same time permitting voice-band and DSL service to be transmitted across the home telephone wiring network.
Despite the advantages of providing DSL, home networking, and POTS signals simultaneously over a common home telephone wiring network, mechanisms are needed to prevent DSL and/or home networking signals from reaching voice-band/POTS appliances coupled to the home telephone wiring network. It is also desirable to prevent POTS device impedance effects from entering onto the home telephone wiring network and potentially degrading the transmission of DSL data signals. Voice-band appliances may include, for example, telephones, answering machines, facsimile machines, V.90 or similar modems, and the like. DSL or home networking signals may cause nonlinear behavior of the voice-band appliances to create noise into the POTS connection.
Voice-band appliances typically undergo impedance changes during operation. In one situation, it has been observed that the change of state from “on-hook” to “off-hook” of the telephone equipment and sometimes the telephone terminal equipment even being “on-hook” can create a resonance effect to occur so as to drop the impedance value to a comparatively low level, even at comparatively high frequencies. Preventing DSL and home networking signals from reaching voice-band appliances also protects the DSL and home networking transports from high-frequency inter-modulation products of the voice-band appliances.
Second-order low-pass Butterworth filters are well known in the signal conditioning and filtration arts, and are commonly inserted between the home telephone wiring network and an associated POTS device to prevent DSL signals (e.g., ADSL signals), on the home telephone network from affecting the POTS device. These devices also act to prevent transient noise from POTS devices from interfering with the proper operation of a DSL modem coupled to the home network, and vice-versa.
The second-order Butterworth filter circuit is inherently asymmetrical and generally includes one coupled inductor (or two uncoupled inductors) and one capacitor. This design requires, for proper operation, that the filter be oriented between the POTS device and the home telephone wiring network such that the coupled inductor is disposed between the home telephone wiring network and the capacitor. If the capacitor is disposed adjacent to the home telephone wiring network, high frequency signals, such as DSL signals, on the home telephone wiring network are likely to short across the capacitor, thereby potentially interfering with the operation of the DSL modem.
Another disadvantage of conventional second-order Butterworth filter designs is that they often do not provide sufficient attenuation of DSL signals. For example, a typical second-order Butterworth filter may be designed with a low insertion loss throughout the pass band (including the POTS band) and has a cutoff frequency above the POTS signal band and below the ADSL transmission band (i.e. below about 20 kHz). The attenuation achieved at the higher ADSL frequencies is generally insufficient, and allows a significant amount of DSL transmit signal leakage through the filter, which can result in interference with the associated POTS device, particularly if the associated POTS device is a data device, such as a modem or facsimile machine.
The T1.421 In-Line Filter Standard of the Standards Committee T1 (Telecommunications) of the American National Standards Institute, also formerly the T1E1.4/2000-110 draft (hereinafter “Standard”) is a standard relating to in-line filters used in the telephone and data communication industries for protection of typical POTS devices when high speed digital services (e.g., ADSL, G.lite, etc.) are deployed on the same telephone line. The Standard is intended to allow complete interoperability between vendors and reduce maintenance of POTS and related equipment. The Standard specifies a variety of specific performance objectives and criteria, including (i) reduced DC impedance through the filter; (ii) enhanced attenuation of higher frequency data signals in the voice band; (iii) the minimization of voice-band transmission effects for up to five (5) installed filters; and (iv) maintaining voice-band signal levels such that operation of currently deployed devices such as V.90 modems are not impaired.
Existing prior art approaches to signal conditioning and filtration (such as the aforementioned Butterworth circuits) fail to meet these more stringent objectives due in part to their less than optimal design process, which does not effectively account for complex impedances on the load and/or source end of the conditioning/blocking circuit(s).
Another desirable attribute of “consumer” filtration and conditioning circuits is low cost of production. Significant factors in determining the cost of producing a filter include (i) the number of components that make up the filter, and (ii) the relative cost and performance of the components used. In general, the higher the number of components that make up the filter, the higher the cost will be to produce the device. Obviously, more components can also impact the space required to accommodate the device, and can necessitate larger housings and internal PCBs. Consequently, it is desirable to keep the component count and component size of a given filter design low to keep the production cost low.
Based on the foregoing, a need exists to provide improved apparatus and methods for preventing signals from DSL and home network signal carriers from reaching voice-band appliances such as telephones, facsimile machines, and modems. Additionally, such improved apparatus would provide isolation of DSL devices and HPNA (Home Phoneline Network Alliance) standard devices from the impedance fluctuations of voice-band appliances. Ideally, such methods and apparatus would be easy and cost efficient to implement and manufacture, respectively. The apparatus would also exhibit enhanced electrical performance, such as for example compliance with more stringent performance standards such the previously described T1.421 standard.